In connection with power transmission by means of high-voltage direct current between more than two converter stations, two different principles are known, namely, series connection and parallel connection, respectively, of the converter stations included in the power transmission installation. In a series connection, which means that all the stations are traversed by the same direct current, the power control is performed by voltage control of each one of the stations. However, in practice, this means a relatively limited control range. In a parallel connection of the converter stations, which means that all the stations are connected to the same voltage, the power control is performed by control of the current through the respective converter. This solution gives greater freedom as regards control possibilities but implies that each one of the stations must be designed for full voltage. This is costly, in particular for those cases where the power which is to be tapped from the dc line is relatively low.
U.S. Pat. No. 4,259,713 discloses a converter station for discharging (i.e. tapping off) power from a dc line in a power transmission system for high-voltage direct current. The converter station comprises an inverter designed for a voltage which is considerably lower than the line voltage and which is connected to the dc line via a direct-voltage converter. The direct-voltage converter comprises, in addition to inductive and capacitive components, diodes and a pulse-controlled thyristor. The thyristor may be provided with an oscillation circuit for extinction or, alternatively, be designed as a gate turn-off thyristor.
The direct-voltage converter comprises a large number of components and must be designed for the whole line voltage, which means that it becomes relatively expensive. Since the direct-voltage converter, from the point of view of insulation, must be designed for the whole line voltage, it is not suited to be designed as an enclosed outdoor device, which in many cases is desirable. In case of a fault in the control of the thyristor or in the thyristor itself, resulting in this becoming continuously conducting, the inverter and the components in the direct-voltage converter which are positioned between the thyristor and the inverter are subjected to an increased direct voltage.